[Feature] Add functionality to use AAD B2C authentication

[AAD-B2C-specifics](https://github.com/AzureAD/microsoft-authentication-library-for-dotnet/wiki/AAD-B2C-specifics)

Please add some overloads to MSALProvider constructor and GetTokenAsync:

AAD B2C needs the bold marked specifics:

application = PublicClientApplicationBuilder.Create(ClientID)
.**WithB2CAuthority(Authority)**
.Build();

and

await application.GetAccountsAsync( **B2CPolicySignUpSignIn** );